AddBuildToChannel not handling builds that are already on a channel correctly

• This issue is blocking
• This issue is causing unreasonable pain

When we call AddBuildToChannel, if a build is already on a channel, we are supposed to return successfully. Today, the code looks like this

            Build build = await _context.Builds.FindAsync(buildId);
            if (build == null)
            {
                return NotFound(new ApiError($"The build with id '{buildId}' was not found."));
            }

            // If build is already in channel, nothing to do
            if (build.BuildChannels != null &&
                build.BuildChannels.Any(existingBuildChannels => existingBuildChannels.ChannelId == channelId))
            {
                return StatusCode((int)HttpStatusCode.Created);
            }

However, build.BuildChannels is always null, because we don’t include BuildChannels when getting the build that should exist. We need to add .Include(b => b.BuildChannels) to the build query.

[premun]

There are actually unit tests for the controller, however the local SQL server doesn’t throw. I opened a PR but I will do additional validation tomorrow to make sure I actually fixed the bug (debugging of the code shows the correct code path).

[riarenas]

I don’t think we have unit testing in place for individual darc commands yet, but this seems like a great candidate for that? I think it’d be overkill to make it part of the scenario tests when it’s just verifying that one command doesn’t throw.